This invention relates in general to switching apparatus and, in particular, to a switching apparatus for use in sectionalizing electrical power distribution by underground power cables.
More specifically, this invention relates to a modularized load break switch carried in a below grade vault which is utilized to interrupt load currents within the rating of the device and to minimize equipment failure and possible subsequent human injury upon any occasion of fault current being coupled into the switch.
Switching equipment carried in below grade vaults is utilized to terminate and sectionalize underground electrical power distribution for routing the distribution of electrical energy during various changing power distribution periods or in the event of interruption to normal power distribution channels. Since the electrical power distribution cables are below grade, the switching systems are installed in below grade vaults and, therefore, must be capable of being totally submerged.
In certain prior art systems, the switching apparatus is carried within the below grade vault and the vault pumped dry, or partially dry, when the equipment is operated. In addition, an oil filled tank permits a more compact vault and switching apparatus due to the increased dielectric strength of oil over air, therefore, increasing the arc extinguishing and absorbing characteristics.
Some oil filled switches are manually operated by the direct coupling of an operating handle to the switching assembly. The operating handle on the outside of the tank is directly coupled to the rotating contacts of the switching system inside the tank and, therefore, the velocity and closing of the rotating contact system is entirely dependent upon the skill of the human operator. These direct coupled systems are prone to cause equipment failure and possible subsequent injury to the operator in the event the switch is improperly operated. Such equipment failure and subsequent operator injury are frequently occasioned when an energized switch is accidentally operated into and of ground position.
Since the load break switches are designed for terminating and sectionalizing electrical power distribution and not for interrupting fault current, the switches are designed to generally handle a symmetrical RMS basis load current of approximately 600 amps or less, and are not designed to interrupt fault currents which may generally run as much as 13,000 to 25,000 amps. In the event an operator closes an energized switch into a ground position, at the instant of contact the operator would feel and hear the flow of fault current and his natural impulse would be to open the switch. The switch would now be required to interrupt fault current which might be twenty to forty times greater than the maximum current load for which the switch is designed. The resulting fault current arc energy which is generated within the vault would generally be sufficient enough to increase the internal tank pressure causing distortion, rupturing and/or an explosion of the switch tank. If interrupting devices such as fuses, circuit breakers, etc. do not clear the fault fast enough switch failure occurs.
In an attempt to overcome equipment failure and possible subsequent operator injury which is occasioned with such type of load break switches, spring mechanisms have been designed to control contact rotation. These spring systems provide sufficient contact velocity and energy for opening and closing the load break switches. They also take the actual movement of the contacts out of the hand of the human operator, and make the closing or opening dependent upon mechanical control. These mechanisms are designed so that rotation of the external operating handle compresses a spring and, upon rotation of the operating handle beyond a predetermined point, the compressed spring energy is transferred to the rotating contact system. As a result, the switch contacts are moved quickly and firmly into their next position, whether it be opened or closed. Due to the speed of the spring energized contact movement on these mechanisms, the external operating handle does not control the switch contact movement. During the opening or closing operation, the compressed spring energy drives the movable contacts faster than the operator can rotate the external operating handle.
With such a properly designed spring loaded mechanism, complete travel of the rotating switch contacts and engagement with the stationary contact is assured regardless of the skill of the operator. Therefore, if an energized feeder is accidentally operated into a grounded system or position, the contacts will rotate to a completely closed position. However, in these systems, if the operator reverses the operation of the external handle immediately after closure of the contacts, for example, as a result of the flow of fault current, the contacts will again be opened. Since the operator's hand would at this point be on the external operating handle when the switch contacts closed into the ground position initiating fault current, the operator's natural instincts would be to immediately rotate the external handle to the open position. In the event, the system fault interrupting devices (fuses, circuit breakers, etc.) had not cleared the fault current before the operator rotated the external handle, the load break switch contacts would be required to interrupt the fault currents which, as previously discussed, are beyond the maximum loading for which the switches are designed.
Another problem heretofore associated with vault type load break switches has been the ease with which the switches may be serviced. Heretofore, the switching contacts, both rotating and stationary, were attached by weldments made to the inside of the tank surfaces. Generally, the stationary switch contacts were attached to the front and back walls of the tank, and the switch components assembled within the tank. The spring mechanisms for opening and closure were attached to the front tank walls, or attached to a shaft and oil seal which in turn were ridgidly secured to the front wall of the tank. The rotating switch contacts had one end attached to the drive mechanism, and the other end of the rotating contact system was generally pivotally secured to a weldment on the rear vault wall.
The load break switches were then filled with oil allowing a sufficient space above the oil for air (or gas) to compensate for oil expansion due to temperature increases. Generally, it is desirable to have the air space as small as possible (usually 15% of tank volume at 25.degree. C.) in order to k-ep minimum vault size while allowing sufficient space such that the pressure which occurs in the sealed tank due to the expansion of the oil is controlled within predetermined limits. A preferred installation procedure is to fill the tank to the proper level, purge the air space with dry nitrogen, and then leave a slight positive pressure of nitrogen over the oil. The air pressure within the sealed tank will then range from negative to positive such that under no load and minimum ambient temperatures the air space pressure is negative upon maximum load and maximum ambient temperature the air space pressure becomes positive within a controlled amount.
During pressure variations within the tank, the tank walls deflect and the amount of deflection is a function of the tank size, wall thickness, reinforcing, oil volume, air space and other such exemplary factors. Switch designs which utilize the tank wall as an integral part of the switch must limit the maximum allowable pressure within the tank to the point where tank wall deflection will not cause mechanism malfunction, constraint or contact misalignment while the unit is in service.
One attempt to control pressure has been to utilize pressure relief devices to limit the pressure occuring in the space above the oil. However, such devices add to the unit's cost and, if improperly designed or installed, can permit moist air or free water to enter the vault. This, of course, reduces the dielectric qualities of the oil and can lead to switch failure.
The utilization of a larger air space would decrease the pressure somewhat, but results in an overall increase in tank size adding to unnecessary costs. In addition, many installations because of vault size and vault access openings require a predetermined size of tank.